GPS Position Timeseries

Expanded documentation describing parameter options for the GPS time series web service is provided below. The parameter options are described in the following sections, for example ("analysisCenter") describes the Analysis Center options.

Analysis Centers (“analysisCenter”)

GPS time series position solutions are available from four different Analysis Centers (AC's): PBO (default), NMT, CWU and UNR.

The GAGE GPS AC’s at New Mexico Tech (NMT) and Central Washington University (CWU) analyze data and generate products from more than 2,000 GPS stations in the PBO, COCONet, TLALOCnet and SCIGN networks as well as NGS CORS and other regional stations in middle and eastern North America. The NMT AC uses GAMIT/GLOBK analysis software. The CWU AC uses GIPSY/OASIS analysis software. The GAGE GPS Analysis Center Coordinator (ACC) at MIT generates a combination solution based on the individual NMT and CWU solutions. These three time series are referred to as PBO, NMT and CWU solutions, respectively, and correspond to the PBO, NMT and CWU AC parameter options in the web service. The combined “PBO” solutions are considered to the be standard GAGE data product. Additional information regarding GAGE analysis methods and products is available here.

GPS time series solutions generated by the Nevada Geodetic Laboratory at the University of Nevada at Reno (UNR) are also available. The UNR AC analyzes data from over 13,000 GPS stations around the world using GIPSY/OASIS software. Additional information regarding UNR analysis methods and products is available here. These products are made available through “Plug & Play GPS”, a NASA ROSES ACCESS funded project awarded to UNR and UNAVCO. Additional information regarding the PLUG & PLAY GPS project is available here.

Time series from the GAGE AC’s (PBO, NMT, CWU) and the UNR AC are not directly comparable due to various differing analysis methods. For example, the GAGE and UNR AC’s generate solutions in different reference frames. Another primary difference is that scale changes are not estimated in the GAGE analyses, but UNR analyses do remove scale differences while aligning to reference frames. This can result in differences in the height estimates between the GAGE and UNR solutions as described here.

GPS station time series case studies and other technical news topics are available here.

Reference Frame Information (“referenceFrame”)

GPS time series solutions are available in multiple reference frames dependent on the Analysis Center (AC). The current global reference frame used by all AC’s is IGS08, based on ITRF2008. Solutions in North America fixed reference frames are also available from all AC's but GAGE (PBO, NMT, CWU) solutions use "NAM08" while UNR solutions use "NA12". Solutions are also available in the discontinued reference frames IGS05 and SNF01, but solutions in these frames do not extend past 2014 and are provided for legacy purposes only.

The “referenceFrame” parameter selected for this web service must correspond to the selected “analysisCenter” parameter. For example, “PBO” and “NAM08” is a valid combination but “UNR” and “NAM08” is invalid and the web service will return an error. Valid combinations are as follows:

IGS08: all

NAM08: PBO, CWU, NMT

NA12: UNR

IGS05: PBO, CWU, NMT (discontinued reference frame)

SNF01: PBO, CWU, NMT (discontinued reference frame)

NAM08 is the current North American reference frame for GAGE solutions. NAM08 is realized by rotating ITRF2008 to North America using the Euler vector published by Altimimi et al. (2012). Compared to SNF01, the former North American reference frame used by the GAGE AC’s, NAM08 is better defined, more up to date and does not have errors from relative phase center models or GIA modeling. Further details regarding NAM08 are available here.

The current North American reference frame for UNR solutions is NA12. Details regarding NA12 are provided in Blewitt et al (2012).

Results Content ("report")

There are currently two templates for how the amount of information included in web service results. "Long" (default) will output all content from the time series source file including cartesian and geodetic coordinates and all associated error estimates and covariances. Example source file.

The "short" option will output a simplified version of the source file which only includes north/east/vertical offsets (and standard deviations) relative to a reference coordinate. The short option closely matches the standard csv files available from the UNAVCO ftp site, more so when the “refCoordOption” of the web service is also set to “first_epoch”. Example of source standard csv file.

Reference Coordinate ("refCoordOption")

Station position offsets in the source file are reported relative to a reference position defined by the Analysis Center. The position offset of the first epoch in the source file is typically non-zero. The “refCoordOption” allows the position offset of the first epoch to be adjusted so that it is 0.000m. Having the time series start at zero for all components can facilitate some time series interpretation methods. The actual relative station offsets through time are the same regardless of option selected.

Acknowledgements and Attribution

The GAGE GPS Analysis Centers process data from more than 2,000 GPS stations. Most of these stations are operated by UNAVCO as part of the PBO, COCONet, TLALOCnet and smaller regional networks. Also processed are stations from the Southern California Integrated GPS Network (SCIGN), the NASA Global Geodetic Network (GGN), the International GNSS Service (IGS) network, the Rio Grande Rift network, the GPS Array for Mid America (GAMA), the Basin and Range Geodetic Network (BARGEN), the Idaho National Laboratory (INL) network, the Pacific Northwest Geodetic Array (PANGA), the Western Canada Deformation Array, SuomiNet, GulfNet, and stations near the epicenter of the 23 August 2011 M5.8 Mineral, VA earthquake. Data from GPS stations not archived by UNAVCO are obtained from the NOAA National Geodetic Survey (NGS) Continuously Operating Reference Station (CORS) data center, the NASA Crustal Dynamics Data Information System (CDDIS), the U.S. Geological Survey, the Scripps Orbit and Permanent Array Center (SOPAC), and the International GNSS Service (IGS).

GPS time series solutions generated by the Nevada Geodetic Laboratory at the University of Nevada at Reno (UNR) are based on analysis of over 13,000 GPS stations around the globe. Additional information regarding UNR analysis methods and products is available here.